The present invention relates generally to improvements in controlling the inclusion morphology in steel, and more specifically to practices and compositions which make it possible to achieve shape control of non-metallic inclusions, particularly sulfides, in an improved manner.
It is recognized that the shape of sulfide inclusions affects the physical properties of steel. Elongated, stringy sulfides adversely affect transverse mechanical properties such as ductility, formability, toughness, etc., whereas these properties are improved by a morphology characterized by the presence of globular or blocky sulfides. A conventional method of obtaining the desired morphology in aluminum killed steels has been to add a rare earth metal or a mixture of rare earth metals, e.g., mischmetal, to the ingot molds during teeming. In one typical practice, a mischmetal addition is made to the molds in an amount of one pound per ton of steel. The sulfide shape achieved by mischmetal treatment is predominently globular or spherical.
It has also been proposed to use either zirconium or titanium instead of mischmetal as the sulfide shape control agent. Another proposal in the literature has been to employ mischmetal and calcium in combination as a mold addition.